The market for metal containers has developed widely in recent years due in part to the introduction of the easy open end. While the easy opening feature has been shown to be both effective and convenient, it has given rise to a collateral problem of littering, since traditionally the easy opening feature was pulled free from the container to expose the contents and then subsequently discarded. The indiscriminate disposal of the pull tabs is not only unsightly, but can also present a safety hazard. Further, the relatively small size of the separated tab presents a challenge to those concerned with the collection and recycling of waste material. The solution to this problem resides in a non-detachable tab and preferably a tab which is compatible with the end closure and is easily recycled.
While the industry has addressed this problem by developing a number of closure structures, one of the more successful has been a can end which employs a retained lever tab. In this structure, the tab is joined to the container end by a tongue or flap which is lanced in the web of the tab, and which serves as a hinge or connecting strip. Typically, the tip of the tongue is apertured and staked to end by means of a rivet integral with end. Since the tab functions by levering open a scored portion of the end panel, it must be sufficiently rigid to prevent distortion of its curled rim when leverage is applied to the lift end of the tab to effect rupture and displacement of the scored panel. At the same time, the tongue of the tab must be sufficiently bendable to permit the user to open the container and subsequently bend the tab back out of the way into a position of repose. The situation is aggravated by the user's inclination to fatique the tab by bending it back and forth in an effort to free the tab from the container in accordance with past practice.
Typically, tab rigidity is provided by use of heavy aluminum tab stock and by curling the edges of the stock to form a tab rim. Such a structure, however, results in a tab tongue which is one thickness of metal, lacks bendability and is prone to fracture when subjected to repeated bending, as may be encountered when it is pivoted forward to open the end and then subsequently bent backward to fold the tab out of the way. Fracture of the tongue generally occurs between the terminal ends of the lance and the rivet hole or tongue aperture.
The lack of bendability of the tab tongue can be overcome in part by a more elaborate tab structure, wherein a retaining strip of plastic or ductile aluminum is interleafed into the folds of the tab nose, thereby providing a flexibility and a fracture resistant linkage between the tab and the end. Such a structure requires insertion of a premium tie strip and appreciable press time for fabrication.
Alternative structures have been proposed using a soft ductile alloy for the tab, but with a more complex configuration as a means for achieving rigidity. In another instance, tin-plated or zinc coated steel has been suggested as a material of construction.
The problems of material recycling and raw edge corrosion or alternatively of increased manufacturing cost, which are attendent with these structures, are not readily resolved. Therefore, an easy opening end closure having a central panel wall, a peripheral flange which joins via double seaming to a container is manufactured with an integral rivet and a displaceable panel located radially outwardly of the rivet, and being defined by a score line. For opening there is a lever tab held to the end panel by the rivet such that opposite ends of the tab are apart from the rivet. There is an opening tab nose end and a tab lifting portion with a central web therebetween. The central web is lanced to form a tongue and the tongue is apertured to receive the rivet.
Portions of the tongue are coined to control the mode of bending of the tongue when the tab is pivoted forward about the rivet such that the nose end contacts the displaceable panel. The exact details of the coining position and location are disclosed in the Langseder U.S. Pat. No. 4,211,335 and in the Radtke U.S. Pat. No. 4,210,257. The nondetachable tab lever is disclosed in U.S. Pat. No. 3,967,752 and the integral rivet construction is disclosed in U.S. Pat. No. 3,361,102. The present disclosure relates to techniques for manufacturing the coined tab tongue in a high speed commercial operation. It is recognized that the coining operation reduces the thickness of the metal while improving its strength by encouraging bending along preferred lines or in preferred areas. Finally, it is advantageous to manufacture the tab by feeding a strip of tab stock into the progressive tab die of a press, in which the following sequence of operations is carried out at speeds producing 500 tabs per minute. In a first progressive die station the rivet aperture is punched in the strip, which is then in a second station panelled to form the web of the tab, with the rivet aperture included in the web toward the nose end. In a third station the strip is sheared outward of the web to form a tab blank with a nose end and a lift end, but the tab blank is carried by a tie piece remaining between it and the strip at the center of the lift end. In a fourth station the edges of the blank are wiped to curl the tab rim. In a fifth station the web of the tab is lanced to form a tongue with the aperture in the tongue tip and the tongue root formed proximate the tab nose. In a sixth station the tongue is displaced from the web plane by the lacing, and the portion of the tongue proximate the root is reinserted into the web plane whereas the portion proximate the tip remains out of the web plane. In the seventh station the coining takes place when the tab is struck to form the band of thinned metal circumscribing the aperture. In the eighth station the curling of the rim is completed and the aperture repunched to the desired size and orientation.
The coining operation displaces metal outwardly since the change in thickness of the stock is accompanied by an increase in size of the panel in the plane normal to the direction of the coining. Such a condition is of concern with respect to jamming in the progressive die station wherein the coining of the aluminum lanced tab tongue is performed. Various approaches have been tried to control the position of the tongue during coining. The cutout for the rivet island could be increased to eliminate the need for offsetting the tab tongue rivet island and removal of more metal in the cutout is preferred over spreading of the rivet island to the sides of the tab tongue. The cutout piece of the metal would have to be removed from the die station and it would be difficult to handle a small sliver of metal in a high speed progressive die. For example, piloting the tongue by means of a pin through the rivet hole is unacceptable because during coining the expansion of the metal tends to close the hole and lock the tab to the pin. A rubber clamp pad positioned to hold the tab during coining has too much give in the plane of the coining particularly when the coining is not symmetric and the forces of the tooling tend to shove the tab tongue sideways. No positive means of holding the tab tongue sufficiently to control the location of the coining was apparent and since the tongue is held by the remaining uncut metal which ultimately becomes the hinge for the tab and the tab is held in the progression by a fragile tie located at the left end of the tab. Consequently, the tab is free to swing relative to the strip of the progression and the tongue is capable of being moved sideways relative to the web of the tab. The progressive die used to totally form the tab must be located with respect to the tab during each operation. In the past the criticality of position was only with respect to the tab but with coining the position of the tongue is particularly critical.
It is recognized that the coining operation will expand the metal of the tongue area of the tab in the horizontal plane (relative to the tooling axis) such that the tongue would lock or jam the tooling. If the tongue jams, stripping becomes difficult and prevents high speed manufacture. While the coining is valuable for the reasons stated in the prior art, it is impossible to produce the preferably coined tab tongue without an improvement to the tooling.